Research on myotonic dystrophy type 1 (DM1) has led to the recognition of a new mechanism for genetic disease. Contrary to a central dogma of human genetics, in DM1 it is the transcription product of a mutant allele that interferes with cell function and triggers a disease state, independently of the protein it encodes. During the initial period of support under this K24 Award, the applicant, his mentees, and their collaborators had a leading role in establishing this RNA-mediated disease process, and elucidating the mechanism by which it occurs. Their findings indicate that expression of RNA with an expanded CUG repeat (CUGexp) leads to sequestration of muscleblind (MBNL) proteins in nuclear foci. This depletion of MBNL proteins from the nucleus results in misregulated alternative splicing, or spliceopathy, that underlies the symptoms of DM1. Having identified viable targets for therapy in DM1, the applicant is now proposing to redirect the main focus of his research group toward developing treatments. A period of protected time and intense effort will be required to acquire the skills and collaborative relationships that are necessary to make this transition. In so doing, the applicant will create a training environment that is ideal for new investigators who are interested in research translation as applied to neuromuscular diseases. The preliminary data indicate that oligonucleotides binding to CUGexp RNA can release sequestered proteins, restore normal patterns of alternative splicing, and improve the physiological derangements in a transgenic mouse model of DM1. These data provide proof-of-principle that protein sequestration is a pivotal event in DM1 pathogenesis and that phenotypes in DM1 are reversible if sequestered proteins are released. The first Aim of this proposal is to develop and perform high throughput screens for small molecules having similar effects, or for compounds that reverse spliceopathy by other means. The second Aim is to develop the therapeutic potential of antisense oligonucleotides as a strategy for releasing the proteins that become sequestered in DM1. Anticipating that an era of rationale therapy.for DM1 is rapidly approaching, the third Aim is to develop and validate an assay for spliceopathy as a biomarker for evaluating therapeutic effects in clinical trials for DM1.